Conventional agricultural balers comprise a frame which is travelled on a pair of wheels over a field for picking up therefrom hay, straw or silage grass and feeding this crop material to a baling chamber in which it is compressed to parallelepiped packages under action of a plunger which reciprocates inside the baling chamber. When the packages have reached a predetermined length a tying mechanism is operated to encircle the completed package with a plurality of strands and to knot the strands ends together to form a finished bale which will be ejected out of the baler.
Commonly the tying mechanism is actuated by a trip arm mechanism comprising a sensor wheel which is rotated by the crop material advancing in the baling chamber and a spring loaded trip arm which is raised by the shaft of the sensor wheel until this shaft reaches a notch in this arm. The trip arm is then pulled forwardly and this motion is transmitted upon a lever for engagement of a dog clutch mechanism which accomplishes one full revolution of a drive shaft of the tying mechanism.
The operation of the tying mechanism involves the movement of needles across the baling chamber to provide the lengths of tying strands to the knotter units which tie these lengths to the ends of the same strands to create closed loops around the crop packages and form a finished bale.
As the movement of the plunger is not suspended during the actual tying cycle, careful timing of the tying mechanism is required to preclude collision of an incoming batch of crop material which is pushed rearwardly by the plunger, and the needles. Such collision inevitably engenders serious damage to the needles themselves or to the needle mounting structure.
In most balers, the timing is realized through a fixed transmission between the plunger crank mechanism and the hub of the dog clutch. The orientation of the cam member, which is affixed to the hub for engagement by the dog transmitting the movement from this continuously rotating hub upon the shaft of the tying mechanism, is essential for accomplishing a good timing of the tying cycle.
It has been experienced that, under certain circumstances, after a normal start of the tying cycle, the motion of the needles and knotters may be interrupted while the plunger still continues to reciprocate inside the baling chamber. This may be caused by an incomplete engagement of the dog and the cam member of the clutch. E.g. when the trip mechanism releases the dog at a moment that the cam passes along the dog, the latter may contact the edge of the leading flank of the cam member. The dog then may be kicked inwardly to re-engage the hub somewhat further and, after one revolution of the hub, the dog may contact the full flank of the cam and be entrained by the same to accomplish a proper revolution of the knotter shaft. However, it is also possible that the dog rests for a while on the edge of the cam corner and starts entraining the knotter shaft, thereby initiating the tying cycle and raising the needles, and then unexpectedly slips off the cam. The knotter shaft is halted and the needles remain in their raised position wherein they eventually are engaged by fresh crop material at the next plunger stroke. The plunger forces break the needles or, at least, distort the needle mounting.
Hence it is paramount that upon actuation of the trip mechanism there remains a positive binding between the dog and the cam of the clutch mechanism until the revolution of the clutch is completed. In the dog clutches disclosed in German Patents No. DE 594 733 and No. DE 622 348, incomplete binding of the dog and the cam member is precluded by an extension finger on an arm of the clutch dog. A guide member is attached to the rotating hub adjacent the cam for engagement by the finger in case the clutch mechanism is tripped while the cam is passing in the vicinity of the dog. The dog is released but the finger slides along the guide member and keeps the dog remote from the cam. As the guide member is rotated further, the finger is released and the dog assumes its outer position against the inside of the hub. After a complete revolution the guide member passes over the finger without contacting the same and the dog is permitted to engage the flank of the cam on the inside of the hub.
Such system may be quite successful in assuring a proper binding between the dog and the cam of the dog clutch, but it is not really satisfactory as the critical moment for erratic actuation of the clutch now has shifted to the moment the front edge of the guide member passes along the extension finger of the dog. When the clutch mechanism is tripped at such moment, the edge may engage the top of the finger and start rotating the dog and knotter shaft. Eventually the finger slips off the thin edge of the guide member and the knotter mechanism is halted before the knotting cycle is completed and the needles have returned to their home position, out of the trajectory of the reciprocating plunger. Even if the dog and the knotter shaft do not start cycling and the finger immediately slips off the guide member edge, the premature wear of finger and/or guide member will render this protection system unreliable and ineffective.